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Goal: Deliver Benefits to the Public Designing culverts for tomorrow’s hurricanes and extreme storms

Most of the 40,000 stream crossings managed by the Forest Service are in small headwater watersheds. A new study can help engineers estimate how big culverts and other road crossings should be. During floods and times of very high streamflow, improperly sized culverts may fail, leading to increased flooding, soil erosion, economic loss, and disruption of stream connectivity critical for aquatic organisms.

A culvert allows stream water to flow under a roadway
It is important to evaluate the flood carrying capacities of road culverts with appropriate methods that consider extreme precipitation events. USDA Forest Service photo by Devendra Amatya.

The Forest Service manages 600,000 km (372,000 miles) of roads and 40,000 stream crossings. Drainage structures at stream crossings vary depending on the size of the stream and other factors. However, in many cases culverts are undersized.

A new study will help engineers design drainage infrastructure that can accommodate extreme flow events. The study evaluated different methods for estimating precipitation intensity-duration-frequency—a metric used to forecast floods and design drainage structures.

Researchers focused on four watersheds in three experimental forests in Arkansas, North Carolina, and South Carolina. Each watershed had long-term precipitation and streamflow data. Their elevation varied, ranging from the coastal plain to the mountains, and the sites have different drainage areas and annual average rainfall.

The team evaluated two methods used in culvert design to estimate peak discharge rates with long-term site data. They found that in smaller structures, the Rational Method does best at describing 25-year and greater flow events. In larger forested watersheds, the U.S. Geological Survey’s Regional Regression Equation is recommended. Additional validation in watersheds of different sizes and topography is needed.

When a culvert fails, the costs can be very high—both in terms of dollars and ecology. The study suggests that the consequences of structural failure are often much higher than any overdesign costs.

Principal Investigators
Devendra M. Amatya, Research Hydrologist
Peter V. Caldwell, Research Hydrologist
Stephanie Laseter, Biological Scientist, SRS Experimental Forest Network Lead
Johnny M. Grace III, Research Engineer
Ying Ouyang, Research Hydrologist
Ge Sun, Research Hydrologist
Dan A. Marion - SRS (Retired)
Jim Vose - SRS (Retired)
4353 - Center for Forest Watershed Research
4855 - Center for Integrated Forest Science
4854 - Eastern Forest Environmental Threat Assessment Center
Strategic Program Area
Water, Air, and Soil
Estimates of precipitation IDF curves and design discharges for road-crossing drainage structures: Case study in four small forested watersheds in the southeastern U.S.
Research Partners
Steven Schnetzler - Southern Region
John Campbell - Northern Research Station
Sherry Johnson - Pacific Northwest Research Station
Kelly Elder - Rocky Mountain Research Station
External Partners
Shiying Tian - North Carolina State University
Mohamed Youssef - North Carolina State University
George M. Chescheir - North Carolina State University
Sudhanshu Panda - University of North Georgia